Donated human organs are transported for transplant. Scientists hope the new technique could help to alleviate the current shortage of organ donors.
Photograph: Sean Smith for the Guardian

Scientists attempting to grow human organs inside pigs will have to overcome significant hurdles before transplants can take place, geneticists have warned.

In a Panorama documentary showing on BBC1 on Monday night, researchers at the University of California, Davis, reveal how they have injected human stem cells into pig embryos to explore the possibility of growing a human pancreas inside a pig. The research could help to solve the current shortage of organs for transplant – a situation that, according to the NHS, leads to around three deaths a day in the UK.

“I think this is exciting because it is a sensible, practical application of revolutionary new science in the form of induced pluripotential stem cells and gene editing and I think it is very exciting to bring them together, said Sir John Burn, professor of clinical genetics at Newcastle University. “But I also know it is a long way from exciting ideas to the clinic.”

But this time, researchers are hoping that their approach will eventually lead to the possibility of growing human organs suitable for transplant.

To create the “chimeric” embryos, the scientists used a gene-editing technique known as Crispr to knock out a section of the pig’s DNA necessary for the embryo to develop a pancreas. They then injected human induced pluripotent (iPS) stem cells into the pig embryo. These are cells that have the potential to develop into any tissue type in the resulting foetus. Although genetically foreign, they are not rejected by the pig embryo because its immune system has not yet developed.

The human cells would be expected to follow the chemical cues from the pig embryo to develop into different tissues in the foetus. In most cases, they are outcompeted by the pig embryo’s own cells but in the case of the pancreas there are no pig cells to compete with. Hence the embryo goes on to develop a pancreas derived from the injected human cells. After 28 days, the experiment is terminated and the scientists investigate the tissue to see how the embryo is developing and explore what the human cells are up to.

“You are basically creating a vacuum, a hole, so that the human cells respond to the right cues; they make a pancreas. The pig cells can’t. But what we don’t know, and this is what they need to look at, is whether the human cells can also contribute substantially to other tissues, and particularly they are worried about the brain,” said Robin Lovell-Badge, a geneticist at the Francis Crick Institute in London.

It was reported earlier this year that scientists had begun attempts to create the embryos, but in September last year, the US National Institutes of Health said it would not back research into “chimeras” produced from embryos until it knew more about the implications. It cited fears that the presence of human cells could affect an animal’s brain and behaviour, potentially making it more human.

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Pablo Ross, the reproductive biologist from the University of California, Davis, who is leading the research, sought to calm those fears, saying there was a “very low potential for a human brain to grow”.

“Our hope is that this pig embryo will develop normally, but the pancreas will be made almost exclusively out of human cells and could be compatible with a patient for transplantation,” he said.

Lovell-Badge added that there were other problems to consider. While the researchers hope that growing human organs inside a pig using induced pluripotent stem cells from the human recipient could reduce the risk of organ rejection, there could be complications. “There are other cell types that are going to be present in the pancreas which come from the pig – including blood vessels,” said Lovell-Badge. “Those would be a big problem and they would be rejected by a human.” It is also possible that the surface of the human cells might be modified inside the pig embryo, also potentially leading to rejection.

Peter Stevenson, from Compassion in World Farming, told the BBC’s Panorama programme: “I’m nervous about opening up a new source of animal suffering. Let’s first get many more people to donate organs. “If there is still a shortage after that, we can consider using pigs, but on the basis that we eat less meat so that there is no overall increase in the number of pigs being used for human purposes.”

George Church, professor of genetics at Harvard Medical School, who has led similar research into the possible use of chimeras, told the broadcaster: “It opens up the possibility of not just transplantation from pigs to humans but the whole idea that a pig organ is perfectible.

“Gene editing could ensure the organs are very clean, available on demand and healthy, so they could be superior to human donor organs.”